The present invention relates to microlenses formed on the ends of optical fibers for coupling sources of elliptically-shaped radiation such as laser diodes to conventional circularly symmetric single-mode optical fibers.
Microlenses formed on the ends of optical fibers are employed to couple light from sources such as laser diodes to the fiber. Typical 980 nm laser diodes emit a beam having an aspect ratio between 2.5:1 and 4:1. The coupling efficiency between such laser diodes and conventional single-mode fibers is affected by the aspect ratio of the radiating area of the diodes. A coupling loss is induced since the elliptically shaped laser mode must be transformed into an appropriately sized, circularly symmetric fiber mode. Therefore, a meaningful comparison between the efficiencies of different shapes of fiber microlenses can be made only by assuming the same type light source. Coupling efficiency is also affected by fiber-laser alignment. It is therefore judicious to compare the coupling efficiencies of fiber lenses by computer modeling techniques that assume perfect alignment of each lens shape analyzed. Efficiencies expressed herein are based on a computer modeling technique that is similar to that disclosed in U.S. Pat. No. 5,011,254 in that both modeling techniques perform overlap integrals to determine the coupling. The technique employed for the lenses disclosed herein assumes a light beam aspect ratio of 2.45 .mu.m.times.0.71 .mu.m, an optical fiber mode field of 2.0 .mu.m and a wavelength of 980 nm.
Rotationally symmetric lenses have been used for efficiently connecting sources with essentially circular light beams to circularly symmetric single-mode optical fibers. One such scheme for coupling light from a light beam having an aspect ratio of 1.1:1 has been reported to provide a coupling efficiency of 90%. However, for typical laser diodes (aspect ratio of about 3.5:1) the maximum coupling efficiency to a circularly symmetric lensed fiber is about 65-70%. The aforementioned computer modeling technique predicted an efficiency of 69% for a single cone lens.
The double-conical lens (shallow slope at fiber core and steeper slope at fiber cladding) disclosed in U.S. Pat. No. 5,200,024 is said to provide a relatively broad range of coupling efficiencies between 65% and 80%. The aforementioned computer modeling technique predicted an efficiency of 71% for the optimum rotationally symmetric lens. Thus, a theoretical improvement in coupling efficiency of no more than about 2% can be expected by employing a double cone lens rather than a single cone lens. Because of the increased difficulty of making the double cone lens, it may not be practical to employ a lens of such a design to achieve an increase in coupling efficiency of only up to about 2%.
Anamorphic (circularly nonsymmetric) lenses provide more efficient coupling between laser diodes and circularly symmetric single-mode fibers. For example, a wedge-shaped lens (wedge angle .theta.=25.degree.) exhibiting a measured coupling efficiency of 47% is disclosed in the publication S. S. Virendra et al. "Efficient Power Coupling from a 980-nm Broad-Area Laser to a Single-Mode Fiber Using a Wedge-Shaped Fiber Endface", Journal of Lightwave Technology, vol. 8, No. 9, September 1990, pp. 1313-1318. The angle .theta. is the angle made by the intersection of a wedge face with a plane perpendicular to the fiber longitudinal axis. The aforementioned computer modeling technique predicts an efficiency of 89.2%.
Asymmetrical hyperbolic microlenses have been formed on the ends of optical fibers by a laser machining technique. Coupling efficiencies of -0.75 dB (about 84%) have been reported with lasers operating at a wavelength of 980 nm and having an approximately 3:1 beam ellipticity. Such lenses are very difficult to fabricate.